Ecophysiological trait differentiation among coexisting species is thought to result from the evolutionary adaptation of species to their environment, and it may influence abundance patterns by allowing some species to become better competitors for limiting resources.  Species invasions may alter trait distributions if exotic and native floras are functionally distinct.  Thus, provenance-related trait difference may determine the success of species invasions and the potential for ecological restoration.  By assessing differences in traits between native and exotic species, we can make predictions about which traits which will have strongest impact on the outcome of competitive interactions between native and exotic species.  We examined the origins of trait differentiation and the importance of traits in determining plant abundance using eighteen California grassland species grown in replicated monocultures.  We first determined how well the variation in plant traits corresponded with phylogeny (class, family, and species), provenance (exotic or native), and growth form (annual or perennial).  Then, we tested which traits had the strongest power to predict species abundances along natural resource gradients.  We found that all three sources of trait variation were strongly correlated with plant traits.  In addition, ecophysiological traits accounted for 75-85% of the variability in species abundances in natural communities. This result suggests that an understanding of the origin of species trait variation and ultimately outcome of interactions between native and exotic species in natural and restored communities.